The study of damage to buildings during earthquakes has resulted in buildings which can better withstand damage from an earthquake. Each time an earthquake occurs, such a Loma Prieta in 1989, Northridge in 1994, Koba in 1995 and others, the damage is assessed. The cause of that damage is postulated, and ways to avoid that damage in the future may be also determined. Building codes can be modified to reflect new understandings of the mechanisms of failure.
For example, the recent earthquakes have caused concrete highway overpasses to fail. The Northridge earthquake caused cracking of steel columns in high rise buildings. Loma Prieta and Koba caused liquefaction of soil which itself caused damage. All of these damage mechanisms can be reflected in new building designs and can be used to try to mitigate the damage.
It is simply not acceptable to wait until the next earthquake to get more conceivably life saving information. Hence, the science of earthquake simulation has evolved. This includes the use of computer code to model structures and simulate the response to certain input motions. The large number of different possibilities, however, has made it difficult to model this satisfactorily.
A more real-time system forms a physical model of a structure or a portion of a structure. That structure is built on a shaking table. The shaking table is moved to simulate the earthquake ground motion. The system described in the preferred embodiment defines a new kind of structure shaking system which addresses certain drawbacks and limitations in the current system.
Current shaking tables range in size, with a large table being fifty feet or more. A large machine can be used for testing a small full size building, or a full scale component such as joint. All of the machines have in common a rigid table supported by a very large electro-hydraulic cylinder. The hydraulic cylinder is driven by a computer system to form the motion simulating an earthquake. The dynamic forces move the table as a whole, using thousands of tons of force as necessary to simulate the earthquake.
The nature of the ground motion in an earthquake can complicate this system. The ground moves more or less randomly in the horizontal plane, the x and y directions, as well as in the plane orthogonal to the horizontal plane, the z direction. Rotational movements are also developed in each of the x, y and z directions. This results in a total of six degrees of freedom.
Different simulating systems allow different numbers of degrees of freedom. Each additional degree of freedom greatly increases the complexity of construction and hence the cost. Moreover, each actuator system must inevitably react against the stationary object. This often provides forming large and heavy reaction masses in the nearby ground to prevent the vibration from being transmitted to nearby buildings.